1. Field of the Invention
The present invention relates to a piezoelectric filter including a piezoelectric substrate having first and second major surfaces opposed to each other and at least one side edge surface located between the first and second major surfaces, a plurality of resonating units each including a pair of resonance electrodes provided on the first major surface with a gap defined between the pair of resonance electrodes and a common electrode provided on the second major surface and opposed to the pair of resonance electrodes with the piezoelectric substrate disposed therebetween, and a coupling capacitance unit including a pair of coupling capacitance electrodes provided on the first and the second major surfaces, respectively and opposed to each other via the piezoelectric substrate. The piezoelectric filter can be used as a band-pass filter in a television receiver or a radio receiver or other similar device.
2. Description of Related Art
Recently, there has been great demand to decrease the size of electronic devices such as a television receiver, a radio receiver and the like. As a result, it has also been required that a size of band-pass filters incorporated in such devices also be significantly reduced in size while also requiring either maintaining the capacitance or even providing increased capacitance despite the decrease in overall size of the component.
In order to increase capacitance, a distance between electrodes can be decreased or an area of the electrodes on the major surfaces of a piezoelectric substrate can be increased. The distance between electrodes can be decreased by decreasing the thickness of the piezoelectric substrate but this changes the resonance frequency of the piezoelectric component to a different, undesired frequency. Thus, this method is not acceptable. If the area of the electrodes on the major surfaces of the piezoelectric substrate is increased, this increases the size of the filter which is also unacceptable. These problems will be described in more detail with reference to particular conventional structure in the following paragraphs.
An explanation will be given of the structure of a piezoelectric filter constructed for use in such electronic devices, with reference to FIG. 8. As shown in FIG. 8, a piezoelectric filter 51 includes a substantially rectangular piezoelectric ceramic plate 52 polarized in a thickness direction. A first resonating unit 53, a second resonating unit 54 and a coupling capacitance unit 55 are provided on the piezoelectric ceramic plate 52.
The first resonating unit 53 is provided with a pair of resonance electrodes 53a and 53b arranged to be opposite to each other with a gap defined therebetween on an upper surface of the piezoelectric ceramic plate 52 and a common electrode 53c arranged to oppose the pair of resonance electrodes 53a and 53b on the lower surface of the piezoelectric ceramic plate 52 with the piezoelectric ceramic plate 52 disposed between the resonance electrodes 53a and 53b and the common electrode 53c. The second resonating unit 54 is also provided with a pair of resonance electrodes 54a and 54b and a common electrode 54c which are arranged as described above with reference to the first resonating unit 53.
The coupling capacitance unit 55 is provided with a coupling capacitance electrode 55a located on the upper surface of the piezoelectric ceramic plate 52 and a coupling capacitance electrode 55b located on the lower surface.
In the piezoelectric filter 51, an input electrode 56a is disposed at a corner portion of the upper surface of the piezoelectric ceramic plate 52 and an output electrode 56b is disposed at the vicinity of the other corner portion of the upper surface of the piezoelectric ceramic plate 52. The input electrode 56a is connected to the resonance electrode 53a. Further, the output electrode 56b is connected to the resonance electrode 54b of the second resonating unit 54.
A lead-out electrode 56c which is connected to the ground potential is disposed on the lower surface of the piezoelectric ceramic plate 52. The lead-out electrode 56c is connected to the common electrodes 53c and 54c and the coupling capacitance electrode 55b. Further, the resonance electrode 53b and the resonance electrode 54a are electrically connected via a conductive connection portion 57 and the coupling capacitance electrode 54a is connected to the conductive connection portion 57.
In the piezoelectric filter 51, the coupling capacitance unit is constituted by forming the above-described coupling capacitance electrodes 55a and 55b. A desired filter characteristic cannot be provided unless the coupling capacitance unit is provided with a certain amount of capacitance. Accordingly, the coupling capacitance electrodes 55a and 55b must have large areas of at least a certain minimum size, which has prevented a reduction in size of the piezoelectric filter 51.
In one specific example of a related prior art device, Japanese Unexamined Utility Model JU-A-7-43006 discloses a piezoelectric filter which is illustrated in FIG. 9. According to the piezoelectric filter 61, a reduction in size is achieved by using an externally mounted multi-layer capacitor in order to provide a coupling capacitance. That is, the piezoelectric filter 61 is defined by bonding a multi-layer capacitor 65 on a piezoelectric ceramic plate 64 where a first resonating unit 62 and a second resonating 63 are disposed.
The first and the second resonating units 62 and 63 are arranged similarly to the first and the second resonating units 53 and 54 of the above-described piezoelectric filter 51. A resonance electrode 62b on one side of the first resonating unit 62 is connected to a coupling capacitance electrode 66 and a resonance electrode 63a on one side of the second resonating unit 63 is also connected to the coupling capacitance electrode 66. An external electrode 65a of the multi-layer capacitor 65 is connected to the coupling capacitance electrode 66. Another external electrode 65b of the multi-layer capacitor 65 is electrically connected to a coupling capacitance electrode provided on one surface of the piezoelectric ceramic plate 64 which is not illustrated.
That is, the piezoelectric filter 61 is arranged such that the multi-layer capacitor 65 is connected across the pair of coupling capacitance electrodes provided on the piezoelectric ceramic plate 64 in a parallel arrangement. According to this arrangement, the electrostatic capacitance of the multi-layer capacitor 65 is added and therefore, a larger coupling capacitance can be achieved without increasing the areas of the coupling capacitance electrodes.
However, according to the piezoelectric filter 61, the multi-layer capacitor 65 must be connected to the piezoelectric ceramic plate 64 and therefore, the manufacturing steps are complicated. In addition, the multi-layer capacitor 65 must be prepared as a separate component which is separate from the piezoelectric ceramic plate 64 and which is formed by arranging the first and second resonating units and the coupling capacitance unit as described above. Therefore, the manufacturing cost of the piezoelectric filter is increased.
Moreover, the outer shape of the piezoelectric filter described above is complicated since the multi-layer capacitor 65 is bonded to the piezoelectric ceramic plate 64 as shown in FIG. 8. As a result, when the piezoelectric filter 61 is arranged as described above and shown in FIG. 8, for example, an electronic component which is externally mounted on the substrate via resin, the overall shape of the total apparatus is uneven and therefore, cracks, defects and the like are liable to occur.
Further, even when the piezoelectric filter 61 is arranged such that an electronic component such as a chip capacitor is externally mounted via resin on the filter 61, the overall shape of the resulting apparatus is uneven. Therefore, when a taping process used for joining the electronic component to the piezoelectric filter 61 is performed, the piezoelectric filter may not be mounted reliably and securely in a recess portion of a taping and therefore, taping failure is liable to occur.
Thus, it has not been possible to achieve a decrease in filter or component size and an increase in component capacitance without either increasing the size of the electrodes, decreasing the thickness of the piezoelectric substrate which changes the resonance frequency of the component, and causing a component to have an uneven and unstable shape caused by a separate component being awkwardly mounted thereon.